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(cl)Macro Bindings

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Macro Bindings
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These forms create local macros and "symbol macros."

 - Special Form: macrolet (bindings...) forms...
     This form is analogous to `flet', but for macros instead of
     functions.  Each BINDING is a list of the same form as the
     arguments to `defmacro*' (i.e., a macro name, argument list, and
     macro-expander forms).  The macro is defined accordingly for use
     within the body of the `macrolet'.

     Because of the nature of macros, `macrolet' is lexically scoped
     even in Emacs Lisp:  The `macrolet' binding will affect only calls
     that appear physically within the body FORMS, possibly after
     expansion of other macros in the body.

 - Special Form: symbol-macrolet (bindings...) forms...
     This form creates "symbol macros", which are macros that look like
     variable references rather than function calls.  Each BINDING is a
     list `(VAR EXPANSION)'; any reference to VAR within the body FORMS
     is replaced by EXPANSION.

          (setq bar '(5 . 9))
          (symbol-macrolet ((foo (car bar)))
            (incf foo))
          bar
               => (6 . 9)

     A `setq' of a symbol macro is treated the same as a `setf'.  I.e.,
     `(setq foo 4)' in the above would be equivalent to `(setf foo 4)',
     which in turn expands to `(setf (car bar) 4)'.

     Likewise, a `let' or `let*' binding a symbol macro is treated like
     a `letf' or `letf*'.  This differs from true Common Lisp, where
     the rules of lexical scoping cause a `let' binding to shadow a
     `symbol-macrolet' binding.  In this package, only `lexical-let'
     and `lexical-let*' will shadow a symbol macro.

     There is no analogue of `defmacro' for symbol macros; all symbol
     macros are local.  A typical use of `symbol-macrolet' is in the
     expansion of another macro:

          (defmacro* my-dolist ((x list) &rest body)
            (let ((var (gensym)))
              (list 'loop 'for var 'on list 'do
                    (list* 'symbol-macrolet (list (list x (list 'car var)))
                           body))))
          
          (setq mylist '(1 2 3 4))
          (my-dolist (x mylist) (incf x))
          mylist
               => (2 3 4 5)

     In this example, the `my-dolist' macro is similar to `dolist'
     (Note: Iteration) except that the variable `x' becomes a true
     reference onto the elements of the list.  The `my-dolist' call
     shown here expands to

          (loop for G1234 on mylist do
                (symbol-macrolet ((x (car G1234)))
                  (incf x)))

     which in turn expands to

          (loop for G1234 on mylist do (incf (car G1234)))

     Note: Loop Facility, for a description of the `loop' macro.
     This package defines a nonstandard `in-ref' loop clause that works
     much like `my-dolist'.